1. Field of the Invention
The present invention relates to a DC-DC converter, and more particularly relates to a DC-DC converter that is provided with a bootstrap circuit and that allows selection between synchronous rectification and asynchronous rectification (though the latter is also called diode rectification, in the present specification, “asynchronous rectification” is stacked to).
2. Description of Related Art
In a DC-DC converter that uses an N-channel MOSFET (metal oxide semiconductor field-effect transistor) as a high-side switching device, to turn on the high-side switching device sufficiently, a bootstrap circuit is adopted for stepping up the driving voltage applied to the device's gate to or over the input voltage applied to the device's drain. Such a bootstrap circuit is employed both in so-called synchronous rectification, which uses an N-channel MOSFET as a low-side switching device, and in so-called asynchronous rectification, which does not use an N-channel MOSFET but instead uses a rectification diode. Incidentally, when a P-channel MOSFET is used as a high-side switching device, no bootstrap circuit is required. In that case, however, due to transistor characteristics, the P-channel MOSFET needs to be given a size twice to three times that of an N-channel MOSFET, which is inconvenient. For this reason, not a small number of DC-DC converters employ a bootstrap circuit that has an N-channel MOSFET as a high-side switching device.
Irrespective of whether they are of a step-up type or a step-down type, DC-DC converters can be classified into, for example, those adopting synchronous rectification and those adopting asynchronous rectification. Synchronous rectification involves turning on and off a high-side switching device and a low-side switching device complementarily. As a low-side switching device, an N-channel MOSFET of the same conductivity type as the high-side switching device is used, and asynchronous rectification does not use a low-side switching device but instead adopts, for example, a Schottky barrier diode. Usually, a high-side switching device and a low-side switching device are integrated into one semiconductor chip. Accordingly, synchronous rectification suffers from increased power consumption. On the other hand, in asynchronous rectification, the Schottky barrier diode is provided as a component externally fitted to an IC, and thus does not effect the power consumption of the semiconductor chip. The forward voltage of a Schottky barrier diode is in the range of 0.2 V to 0.5 V, and is thus lower than the forward voltage, 0.7 V, of an ordinary PJ-junction diode. This is beneficial from the perspective of reducing the power consumption of the DC-DC converter as a whole.
According to Japanese Patent Application Publication No. 2010-200554 (hereinafter Patent Document 1), a DC-DC converter is provided which, even when an output voltage is high, can complete initial charging of a bootstrap capacitor and can be started up quickly. Patent Document 1 will be further discussed later.
According to Japanese Patent Application Publication No. 2011-78212 (hereinafter Patent Document 2), in a DC-DC converter, switching between synchronous and asynchronous operation is performed with optimal timing in accordance with the state of a load.
FIG. 5 is a block diagram of a conventional example of a step-down DC-DC converter disclosed in Patent Document 1. This conventional example includes a bootstrap circuit for driving a high-side switching device 51, a switch 60 connected in series with the high-side switching device 51, a reflux diode 53 connected in parallel with the switch 60, a driver 63 for turning on and off the high-side switching device 51, a control circuit 61 for controlling the driver 63, and an oscillator 67 for turning on and off the switch 60.
The oscillator 67 is fed with, via an inverter 66, a run/stop-bar signal, which is an input to the control circuit 61; thus, when the run/stop-bar signal is at high level, the oscillator 67 stops (its output being at low level), and when the run/stop-bar signal is at low level, the oscillator 67 operates (outputting pulses). When the output signal of the oscillator 67 is at high level, the switch 60 is on, so that a charging current passes through a bootstrap capacitor 52 across a route from a regulator 55 to a bootstrap diode 56, to a resistor 57, to the bootstrap capacitor 52, and to the switch 60. When the output signal of the oscillator 67 is at low level, the switch 60 is off, so that the current passing via an inductor L58 decreases as it passes through a body diode (unillustrated) of the high-side switching device 51 to the input IN, until eventually the low potential-side terminal voltage Vsw of the high-side switching device 51 falls to low level.
When the output signal of the oscillator 67 turns back to high level, the switch 60 turns back on, and the bootstrap capacitor 52 is charged. As a result of this sequence of operations being repeated, even when the output voltage Vout does not fall sufficiently, a rise in the terminal voltage Vsw is suppressed so that the bootstrap capacitor 52 can be charged sufficiently.
However, the configuration of the DC-DC converter of Patent Document 1 shown in FIG. 5 requires an extra oscillator 67. This leads to a comparatively increased semiconductor chip area. Moreover, Patent Document 1 is aimed at, when starting up a DC-DC converter, temporarily charging a bootstrap capacitor to achieve reliable and quick start-up, and suggests nothing about charging of a bootstrap capacitor in a steady state of the DC-DC converter. In addition, while suggesting application to both synchronous and asynchronous rectification, Patent Document 1 suggests nothing about switching between synchronous and asynchronous rectification.
According to Patent Document 2, switching between synchronous and asynchronous rectification is possible. The switching, however, requires the provision of an off-state period detector for detecting the absolute value of the off-state period of the high-side switching device, and a control switcher for switching between synchronous and asynchronous rectification based on the absolute value of the off-state period, inconveniently resulting in a somewhat complicated circuit configuration. Furthermore, since the absolute value of the off-state period of the high-side switching device is detected, attempting to cope with varying operating frequencies further complicates the design of the off-state period detector and the control switcher, leading to an even larger circuit scale. In addition, when switched to asynchronous rectification, it is inevitable to use a body diode incorporated on the low-side switching device side and having a comparatively high forward voltage, making reduction in power consumption impossible.